a) Field of the Invention
The present invention relates to a focus detecting device to be used with cameras, with silver halide, video camera, still video cameras and so on.
b) Description of the Prior Art
A large number of inventions have hitherto been made to provide devices for detecting focused conditions of still cameras and so on. Out of the method adopted for these devices, the typical one is to split, with an optical member, an imaging light bundle coming from an object to be photographed into two bundles so as to form two images with parallax on an image surface, and detect an interval between these two images with a photoelectric transducing member for focusing. This method is disclosed, for example, by Japanese Patent Preliminary Publication No. Sho 59-40610. FIG. 1 is a diagram illustrating the principle of the focus detecting device disclosed by this publication, and FIG. 2 is a diagram illustrating the main section of FIG. 1 on an enlarged scale.
In these drawings, arranged on an optical axis O is a primary imaging optical system 3, for example a photographic lens system, for forming a primary image 1 with the light bundle coming from an object to be photographed. In the vicinity of the location at which the image is formed by the primary imaging optical system in the focused condition thereof, i.e., a primary image surface 2 which is positionally equivalent to the light receiving surface of a photographic film or an image sensor, a pupil transmitting optical system 8 is arranged for forming an image of an exit pupil 4 of the primary imaging optical system 3, and an aperture stop 12 having a pair of apertures l and m at locations offset from the optical axis is arranged in the vicinity of a surface 7 on which an image of the exit pupil is to be formed by the pupil transmitting optical system 8. The deflection prisms 13 (only one is shown) are arranged at locations corresponding to the apertures l and m respectively of the aperture stop 12. When the deflection prism 13 is not arranged, the focus detecting optical system functions as an ordinary optical system wherein the imaging light bundles having passed through the upper aperture m and the lower aperture l of the aperture stop 12 form a secondary image at a point P on the optical axis O as indicated by the solid lines in FIG. 2. When the deflecting prism 13 is arranged, in contrast, a virtual image is formed in a condition where the optical axis O is rotated by an angle .theta. around a point Z located at the intersection between the prism insertion surface and the optical axis O. When the deflecting prism 13 is not arranged, the secondary image P is formed taking the primary image 1 as an object point. When the flection prism 13 is arranged, in contrast, the optical function of the focus detecting optical system is shifted by the angle .theta. and a secondary image Q' is formed at a location offset from the optical axis O taking a virtual image Q of the primary image 1 as an object point. When an additional deflection prism corresponding to the upper aperture m is arranged in the direction reverse to the prism 13, a virtual image is formed on the optical axis rotated by an angle of -.theta. and a virtual image of the primary image 1 formed on the optical axis is also formed, whereby two secondary images are formed with parallax. When the position of the primary image 1 is displaced back and forth on the optical axis, the virtual image Q is also displaced on the inclined optical axis. Since the interval between the two virtual images is varied along with the displacement of the virtual image Q, the interval between the secondary images formed with the imaging light bundles having passed through the apertures, i.e., the phases thereof are also varied. Accordingly, it is possible to convert the distance of the displacement of the primary image surface 2 on the optical axis caused by variation of focused position into a shift distance of the secondary image surface 10 in the direction perpendicular to the optical axis O and detect a shift distance for focusing with a photoelectric transducer or the similar element.
Further, Japanese Patent Publication No. Sho 62-33564 discloses a focus detecting device shown in FIG. 3, wherein a field lens 15 is arranged in the vicinity of the primary image surface 2, a positive imaging lens 17 is arranged after the field lens 15 at a location where the front focal point of the positive imaging lens 17 is coincident with the primary image surface 2, and two relay lenses 16.sub.A and 16.sub.B are arranged after the positive imaging lens 17 at locations symmetrical with each other with regard to the optical axis O so that two secondary images 19.sub.A and 19.sub.B are formed with the field lens 15, the positive imaging lens 17, and two relay lenses 16.sub.A and 16.sub.B. The exit pupil surface 4 of a primary imaging optical system (not shown) and the pupil surfaces of the relay lenses 16.sub.A and 16.sub.B are located at positions conjugate with each other with regard to the field lens 15 and the positive imaging lens 17. In the composition of the focus detecting optical system described above, the light bundle transmitted from the primary image surface 2 and having passed through the positive imaging lens 17 is a parallel light bundle. Let us consider an optional point FQ which is located within a detectable field angle on the primary image surface 2, and two light pencils transmitted from said point FQ. After having passed through the positive imaging lens 17, these two light pencils are indicident on the relay lenses 16.sub.A and 16.sub.B at angles .omega..sub.1 and .omega..sub.2 which are equal to each other with regard to the optical axes O.sub.A and O.sub.B of the relay lenses 16.sub.A and 16.sub.B respectively. Further, since the light pencils are parallel with each other in the section between the positive imaging lens 17 and the relay lenses 16.sub.A and 16.sub.B, imaginary apertures 18.sub.A and 18.sub.B which are set by the relay lenses 16.sub.A and 16.sub.B respectively on the surface of the pupil of the positive imaging lens 17 have areas equal to each other. Accordingly, uniformity of illuminance is improved for the image points 19.sub.A and 19.sub.B on the image surface (the secondary image surface 10) of the relay lenses 16.sub.A and 16.sub.B.
Along with the recent progress made in the autofocus mechanisms, there have been developed the photographic elements such as image sensors and silver halide film which have high sensitivities to provide high sharpness. Under this circumstance, it is demanded to develop a focus detecting device which can detect slight deviation between images accurately in the autofocus mechanisms. In order to meet this demand, it is necessary to enhance detecting capability of the photoelectric detecting members such as the photodiode array, but such an improvement is possible only within a certain limited range. In order to obtain higher detecting accuracy, it is necessary to detect a slightly defocused condition as a long displacement distance of images on the light receiving surface of the photoelectric detecting member currently available. For this purpose, it is sufficient to enhance imaging magnification of an optical system for forming a secondary image so that a slight deviation of focused point is presented accurately as a long displacement distance. Further, since an optical system for reimaging the primary image should desirably have a wide field angle, the secondary imaging optical system should ideally have a composition which can provide a wide field angle and a high magnification. In the optical composition of the focus detecting device disclosed by Japanese Patent Preliminary Publication No. Sho 59-40610, however, the optical system for forming the secondary image consists only of a single lens element and the location of the primary image surface 2 is nearly fixed for each photographing optical system, whereby the distance a as measured from the front principal point HF of a secondary imaging optical system 11 to the primary image surface 2 is inevitably determined and has no selectional flexibility. Accordingly, imaging magnification is dependent mainly on the focal length the focus detecting optical system 11, and the lens element composing this optical system must bear all the burden of the power thereof and have a very small radius of curvature. Since the lens element can have an effective diameter at maximum twice as long as the radius of curvature thereof, it is impossible to select so wide a focus detecting field angle for the secondary imaging optical system 11. Since the secondary imaging optical system 11 is composed of a single lens element as described above, it is impossible to select a favorable imaging magnification for the focus detecting optical system. Further, the secondary imaging optical system 11 which is composed of a single lens element makes it difficult to correct aberrations favorably on an image. The aberrations cannot be corrected sufficiently especially when it is desired to obtain a high imaging magnification regardless of the small radii of curvature on both the sides of the lens element composing the secondary imaging optical system 11. Accordingly, imaging performance is degraded, thereby lowering focus detecting accuracy. Furthermore, the composition wherein the secondary imaging optical system 11 is composed of a single lens element restricts design flexibility and does not permit designing the focus detecting device compact by shortening total length of the optical system thereof.
Moreover, Japanese Patent Publication No. Sho 62-33564 discloses a focus detecting device wherein the secondary imaging optical system 11 consists of a plurality of lens elements. However, since this optical system is so composed as to improve uniformity of illuminance on the two images formed on the secondary image surface 10, the light pencils having passed through the positive imaging lens 17 are incident parallely with each other on the relay lenses 16.sub.A and 16.sub.B. In order to obtain a wide field angle on the side of incidence of the secondary imaging optical system 11 and a high imaging magnification on the side of emergence of the secondary imaging optical system 11 in this composition, it is required to independently obtain a wide field angle with the positive imaging lens 17 and a high imaging magnification with the relay lenses 16.sub.A and 16.sub.B. Accordingly, the composition selected by the above-mentioned patent has no flexibility to adjust power distribution though the secondary imaging optical system 11 is composed of a plurality of lens elements. Further, the composition does not permit correcting aberrations favorably or shortening total length of the optical system since the positive imaging lens 17 and the relay lenses 16.sub.A and 16.sub.B must have powers predetermined for obtaining the wide field angle and the high imaging magnification respectively.